The present invention relates generally to devices sensitive to water and/or oxygen, such as organic electroluminescent devices. More particularly, the present invention relates to substrates with improved barrier properties for organic electroluminescent display devices.
Organic light emitting devices (OLEDs) typically comprise a laminate formed on a substrate such as glass or silicon. A light-emitting layer of a luminescent organic solid, as well as optional adjacent semiconductor layers, is sandwiched between a cathode and an anode. The semiconductor layers may be hole-injecting or electron-injecting layers. The light-emitting layer may be selected from any of a multitude of fluorescent organic solids. The light-emitting layer may consist of multiple sublayers or a single blended layer.
When a potential difference is applied across the anode and cathode, electrons move from the cathode to the optional electron-injecting layer and finally into the layer(s) of organic material. At the same time, holes move from the anode to the optional hole-injecting layer and finally into the same organic light-emitting layer(s). When the holes and electrons meet in the layer(s) of organic material, they combine, and produce photons. The wavelength of the photons depends on the material properties of the organic material in which the photons are generated. The color of light emitted from the OLED can be controlled by the selection of the organic material, or by the selection of dopants, or by other techniques known in the art. Different colored light may be generated by mixing the emitted light from different OLEDs. For example, white light can be produced by mixing blue, red, and green light.
In a typical OLED, either the anode or the cathode is transparent in order to allow the emitted light to pass through. If it is desirable to allow light to be emitted from both sides of the OLED, both the anode and cathode can be transparent.
U.S. Pat. No. 5,962,962 describes a basic organic light emitting device. The OLED has a structure in which an anode, an organic light emitting layer, and a cathode are consecutively laminated, with the organic light emitting layer sandwiched between the anode and the cathode. Generally, electrical current flowing between the anode and cathode passes through points of the organic light emitting layer and causes it to luminesce. The electrode positioned on the surface through which light is emitted is formed of a transparent or semi-transparent film. The other electrode is formed of a specific thin metal film, which can be a metal or an alloy.
OLEDs typically have a number of beneficial characteristics, including a low activation voltage (about 5 volts), fast response when formed with a thin light-emitting layer, high brightness in proportion to the injected electric current, high visibility due to self-emission, superior impact resistance, and ease of handling of the solid state devices in which they are used. OLEDs have practical application in television, graphic display systems, digital printing and lighting. Although substantial progress has been made in the development of OLEDs to date, additional challenges remain. For example, OLEDs continue to face challenges associated with their long-term stability. In particular, during operation the layers of organic film may undergo recrystallization or other structural changes that adversely affect the emissive properties of the device.
One of the factors limiting the widespread use of organic light emitting devices has been the fact that the organic polymers or small molecule materials making up the device as well as, in some cases, the electrodes, are environmentally sensitive. In particular, it is well known that device performance degrades in the presence of water and oxygen. Exposing a conventional OLED to the atmosphere shortens its life. The organic material in the light-emitting layer(s) reacts with water vapor and/or oxygen. Lifetimes of 5,000 to 35,000 hours have been obtained for evaporated films and greater than 5,000 hours for polymers. However, these values are typically reported for room temperature operation in the absence of water vapor and oxygen. Lifetimes associated with operations outside these conditions are typically much shorter.
This fault tendency has especially limited the use of mechanically flexible plastic substrates for organic electroluminescent devices, because plastics are generally highly permeable to water and oxygen. Thus, mechanically flexible organic electroluminescent devices have not been available for practical applications.
Some attempts at preventing degradation have focused on removing the heat generated during device illumination. For example, Japanese Patent JP 4-363890 discloses a method in which an organic light emitting device is held in an inert liquid compound of liquid fluorinated carbon. Other efforts have been directed at removing the water that is one of the causes of degradation. JP 5-41281 discloses a method in which an organic light emitting device is held in an inert liquid compound prepared by incorporating a dehydrating agent such as synthetic zeolite into liquid fluorinated carbon (specifically, the same as the liquid fluorinated carbon disclosed in the above JP 4-363890). Further, JP 5-114486 discloses a method in which a heat-radiating layer encapsulating a fluorocarbon oil (specifically, included in the liquid fluorinated carbon disclosed in the above JP-A-4-363890) is formed on at least one of the anode and the cathode, and heat generated during illumination is radiated through the heat-radiating layer to extend the light emission life of the device. However, this method entails additional and difficult manufacturing steps.
Attempts have been made to coat plastics with various inorganic layers to provide a barrier to water and/or oxygen diffusion. For plastic substrates that hold the possibility of being mechanically flexible, the main efforts have involved depositing an inorganic coating such as SiO2 or Si3N4 onto the plastic. However, to date, an adequate system has not been found to prevent degradation of the illumination device. The reason for this is due to imperfections such as pinholes in the inorganic coating. These imperfections provide a path for water and/or oxygen entry. It should be noted that even if an organic coating can be applied without imperfections, imperfections such as cracks often develop during thermal cycling due to the large mismatch in thermal expansion rates for plastics and inorganic coatings.
There are numerous designs of late to minimize water and oxygen diffusion into the active organic electroluminescent device region that have been utilized for rigid devices that do not utilize plastic substrates. One method is to fabricate the device on a glass substrate and then to sandwich it between another glass slide. In this design, because glass has excellent barrier properties for water and oxygen, the weak point in the design is usually the material used to join the glass substrate to slide. Another method described in U.S. Pat. No. 5,882,761 is to fabricate the device on a glass substrate and then to encase the whole device in an airtight chamber filled with a desiccant/drying agent. Another method described in U.S. Pat. No. 5,962,962 is to encase the device in an inert liquid barrier layer.
It would be desirable to provide a substrate with improved barrier properties for organic light emitting devices that could prevent premature deterioration of the elements of the OLED due to permeated water and oxygen without interfering with the light transmission from the OLED. It would also be desirable to provide such a device which was flexible.
Plastic substrates for a device sensitive to water and/or oxygen, such as an organic light emitting device, with increased resistance to water and/or oxygen are disclosed. The plastic substrates comprise a transparent or substantially transparent polymer filled with particles of a getter material having a particle size which is smaller than the characteristic wavelength of light emitted by the organic light emitting device, and thus small enough so as to maintain the substantial transparency of the substrate, generally but not necessarily having a size of less than 100 nanometers (nm).
Also disclosed is a method of protecting a device sensitive to water and/or oxygen comprising applying to at least one surface of the sensitive device, so as to form a seal, a substrate layer comprising a transparent or substantially transparent polymer and getter particles dispersed within the transparent polymer, wherein the getter particles have a particle size which is substantially smaller than the characteristic wavelength of light emitted by the light emitting device, and thus small enough to maintain the substantial transparency of the substrate, generally but not necessary having a size less than 100 nm.